Calculating tension in an elevator

[sweetdaisy186]

Hey guys!

I feel like this should be an easy problem...

Two blocks are fastened to the ceiling of an elevator. Each block has a mass of 10 kg. The elevator accelerates upward at 2 m/s^2. Find the tension in each rope.

My prof drew a picture

 

[sweetdaisy186]

oh whoops, sry I didn't find my question. I thought I could use the T=m*a formula, but that didn't get me the right answer. How do I know which formula to use? Thanks!

 

[SGT]

Have you considered gravity?

 

[sweetdaisy186]

ah! Whoops! I forgot about that! Thanks!

 

[loudgrrl4_ever]

I actually have just about the same problem for an assignment at the moment. The numbers are different, but that doesn't really matter.

I am really stuck.
Since the masses are the same, I am using plain m. Since T=m*a for each rope respectivly, the tension for the top rope should be T=2m*g-2m*a or T=2m(g-a), similarly the tension for the bottom rope would be T=m(g-a).
The problem is, this isn't working.

I am obviously missing something. Any help would be appreciated.

 

[SGT]

I actually have just about the same problem for an assignment at the moment. The numbers are different, but that doesn't really matter.

I am really stuck.
Since the masses are the same, I am using plain m. Since T=m*a for each rope respectivly, the tension for the top rope should be T=2m*g-2m*a or T=2m(g-a), similarly the tension for the bottom rope would be T=m(g-a).
The problem is, this isn't working.

I am obviously missing something. Any help would be appreciated.

Is the elevator going upward or downward?
If it is going upward you should add the accelerations, instead of subtracting.

 

[loudgrrl4_ever]

The elevator is going upward. I don't understand why you would add them, however.
That is the correct answer, but since we have a test tomorrow, I want to be sure I understand it much more than get a point on a silly assignment.

Here's how it is playing out in my mind:
A block is being pulled on upwards directly by the elevator, and downwards by gravity. How does the tension use both of those forces in the same direction?
Thanks for your help :)

 

[SGT]

The elevator is going upward. I don't understand why you would add them, however.
That is the correct answer, but since we have a test tomorrow, I want to be sure I understand it much more than get a point on a silly assignment.

Here's how it is playing out in my mind:
A block is being pulled on upwards directly by the elevator, and downwards by gravity. How does the tension use both of those forces in the same direction?
Thanks for your help :)

Do the balance of forces in the mass. You have mg pulling down and T pulling up, The vectorial sum is ma in the up direction. So, T - mg = ma and T = mg + ma.